In particular, optical power meters which are intended for use as universal measuring instruments, should allow connection of a wide variety of optical waveguides (single-mode fibres, multi-mode fibres, thick-core fibres etc.). In general, the waveguide end portions are accommodated in connector ferrules of widely varying shape and dimensions depending on the connector construction. Mechanical adaptation of various waveguide-connector elements to the measuring-instrument receptacle behind which the photodiode is located, is achieved by means of exchangeable adaptors.
Applied Optics, Vol. 26, pp. 1170-1174 describes the problem that the influence of interreflections between the reflecting surface of the photodiode on the one hand and the end faces of the waveguide connector elements on the other hand differs appreciably and may lead to substantial measurement errors depending on the type of connector. The additional amount of light reaching the photodiode due to interreflections differs for different connector types because the reflection from their end faces which face the photodiodes differs intrinsically. For this reason, interreflections should be avoided or, at least, minimized.
In a device of the type mentioned in the opening paragraph, which is known from Hewlett Packard Journal 1987, pp. 26 and 27, an adaptor is slid onto the end of an optical waveguide which is accommodated in a connector ferrule, which adaptor, in front of the end face of the optical waveguide comprises an aperture plate whose aperture is locally widened to obtain a conical portion (300 .mu.m). A lens directs the emerging light to a photodiode having a surface which is inclined relative to the end face of the optical waveguide. This device is rather intricate. A lens brings about additional reflections.